Add WAF support

[yoink00]

Changes:

• Rebases service-waf-implementation onto master.
• Adds WAF to the new block style code
• Wait for WAF deployments to complete (wait for LastDeploymentStatus to become complete).
• Mark all WAF Configuration optional fields as computed (this indicates that the default values are not known until the Terraform is applied).
• Add an optional field to the test to ensure it is imported correctly and that a plan shows no changes

Acceptance test output (limited to just the WAF tests, updating go.mod to point to a local up-to-date go-fastly):

$ make testacc TESTARGS='-run=\'TestAccFastlyServiceV1WAF.*\''
==> Checking that code complies with gofmt requirements...
TF_ACC=1 go test $(go list ./... |grep -v 'vendor') -v -run='TestAccFastlyServiceV1WAF.*' -timeout 360m -ldflags="-X=github.com/terraform-providers/terraform-provider-fastly/version.ProviderVersion=acc"
?       github.com/terraform-providers/terraform-provider-fastly        [no test files]
=== RUN   TestAccFastlyServiceV1WAFAdd
=== PAUSE TestAccFastlyServiceV1WAFAdd
=== RUN   TestAccFastlyServiceV1WAFAddAndRemove
=== PAUSE TestAccFastlyServiceV1WAFAddAndRemove
=== RUN   TestAccFastlyServiceV1WAFUpdateResponse
=== PAUSE TestAccFastlyServiceV1WAFUpdateResponse
=== RUN   TestAccFastlyServiceV1WAFUpdateCondition
=== PAUSE TestAccFastlyServiceV1WAFUpdateCondition
=== CONT  TestAccFastlyServiceV1WAFAdd
=== CONT  TestAccFastlyServiceV1WAFUpdateCondition
--- PASS: TestAccFastlyServiceV1WAFAdd (55.38s)
=== CONT  TestAccFastlyServiceV1WAFUpdateResponse
--- PASS: TestAccFastlyServiceV1WAFUpdateCondition (111.94s)
=== CONT  TestAccFastlyServiceV1WAFAddAndRemove
--- PASS: TestAccFastlyServiceV1WAFUpdateResponse (106.17s)
--- PASS: TestAccFastlyServiceV1WAFAddAndRemove (152.06s)
PASS
ok      github.com/terraform-providers/terraform-provider-fastly/fastly 264.036s
?       github.com/terraform-providers/terraform-provider-fastly/version        [no test files]

[phamann]

👍 LGTM - we're very excited to get this merged and shipped.

Thank you for all your hard work @oscarDovao @matpimenta & @yoink00 for getting this over the line.